1. Field of the Invention
The invention relates to a power converting device, more particularly to a DC-to-AC power converting device.
2. Description of the Related Art
FIG. 1 illustrates a conventional power converting device proposed in an article by R. J. Wai and R. Y. Duan, entitled “High-efficiency power conversion for low power fuel cell generation system”, IEEE Trans. Power Electronics, vol. 20, no. 4, pp. 874-856, July 2005. The conventional power converting device is adapted for converting a DC voltage input from an external power source into an AC voltage output, such as a sinusoidal signal, and includes a transformer 10, an input diode 13, a clamp circuit 14, a full-bridge circuit 15, and an output capacitor 16.
The transformer 10 includes first and second windings 11, 12 each having a polarity end and a non-polarity end. The polarity end of the first winding 11 is coupled to the external power source. The polarity end of the second winding 12 is coupled to ground.
The input diode 13 has an anode coupled to the non-polarity end of the second winding 12, and a cathode coupled to the polarity end of the first winding 11.
The clamp circuit 14 includes a clamp capacitor 145, first and second clamp diodes 143, 144, and first and second clamp switches 141, 142 each having first and second ends, and a control end for receiving a control signal from a control circuit 17, and each operable between an ON-state and an OFF-state in response to the control signal from the control circuit 17. The first end of the first clamp switch 141 and an anode of the second clamp diode 144 are coupled to the non-polarity end of the first winding 11. The second end of the first clamp switch 141 is coupled to an anode of the first clamp diode 143. The second end of the second clamp switch 142 is coupled to a cathode of the second clamp diode 144. The first end of the second clamp switch 142 is coupled to a cathode of the first clamp diode 143. The clamp capacitor 145 is coupled between a common node (p1) between the second end of the first clamp switch 141 and the anode of the first clamp diode 143, and a common node (p2) between the cathode of the second clamp diode 144 and the second end of the second clamp switch 142.
The full-bridge circuit 15 includes a first series connection of a first diode 151, first and second switches 155, 156, and a second diode 152, and a second series connection of a third diode 153, third and fourth switches 157, 158, and a fourth diode 154. The first and second series connections are coupled in parallel between the cathode of the first clamp diode 143 and ground. Each of the first, second, third and fourth switches 155, 156, 157, 158 has a control end for receiving an external control signal from the control circuit 17, and is operable between an ON-state and an OFF-state in response to the control signal from the control circuit 17.
The output capacitor 16 is coupled between a common node (p3) between the first and second switches 155, 156, and a common node (p4) between the third and fourth switches 157, 158. The AC voltage output is a voltage across the output capacitor 16.
Since the operation of the conventional power converting device is described in detail in the aforesaid article, further discussion of the same is omitted herein for the sake of brevity. The following are some of the drawbacks of the conventional power converting device:
1. Due to the presence of the diodes 151, 152, 153, 154, a loop current is generated and conduction loss is increased when the AC voltage output is switched from one of positive and negative halves of the sinusoidal signal to the other one of the positive and negative halves of the sinusoidal signal.
2. The first and second windings 11, 12 are wound in a complicated sandwich winding manner to enhance a coupling coefficient of the transformer 10. A current flowing through the second winding 12 flows back to the external power source. As a result, energy attributed to the second winding is up to a third of that attributed to the voltage output. Therefore, it is necessary to avoid occurrence of a large loop current that is not introduced to an output side.